Messages in this thread | | | Subject | Re: [PATCH] mm : fix pte _PAGE_DIRTY bit when fallback migrate page | From | Robbie Ko <> | Date | Wed, 15 Jul 2020 10:45:39 +0800 |
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Kirill A. Shutemov 於 2020/7/14 下午6:19 寫道: > On Tue, Jul 14, 2020 at 11:46:12AM +0200, Vlastimil Babka wrote: >> On 7/13/20 3:57 AM, Robbie Ko wrote: >>> Vlastimil Babka 於 2020/7/10 下午11:31 寫道: >>>> On 7/9/20 4:48 AM, robbieko wrote: >>>>> From: Robbie Ko <robbieko@synology.com> >>>>> >>>>> When a migrate page occurs, we first create a migration entry >>>>> to replace the original pte, and then go to fallback_migrate_page >>>>> to execute a writeout if the migratepage is not supported. >>>>> >>>>> In the writeout, we will clear the dirty bit of the page and use >>>>> page_mkclean to clear the dirty bit along with the corresponding pte, >>>>> but page_mkclean does not support migration entry. > I don't follow the scenario. > > When we establish migration entries with try_to_unmap(), it transfers > dirty bit from PTE to the page.
Sorry, I mean is _PAGE_RW with pte_write
When we establish migration entries with try_to_unmap(), we create a migration entry, and if pte_write we set it to SWP_MIGRATION_WRITE, which will replace the migration entry with the original pte.
When migratepage, we go to fallback_migrate_page to execute a writeout if the migratepage is not supported.
In the writeout, we call clear_page_dirty_for_io to clear the dirty bit of the page and use page_mkclean to clear pte _PAGE_RW with pte_wrprotect in page_mkclean_one.
However, page_mkclean_one does not support migration entries, so the migration entry is still SWP_MIGRATION_WRITE.
In writeout, then we call remove_migration_ptes to remove the migration entry, because it is still SWP_MIGRATION_WRITE so set _PAGE_RW to pte via pte_mkwrite.
Therefore, subsequent mmap wirte will not trigger page_mkwrite to cause data loss.
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